This is a divisional application of Ser. No. 846,222.
The present invention relates to novel and improved method for producing heat-resistant materials. More particularly, it relates to a method for producing the materials for the casting equipment of non-ferrous molten metals which have comparatively low melting points such as aluminium, tin, zinc, magnesium etc., especially in regards to a material which constitutes a portion of such equipment that is directly in contact with the molten metal.
For low melting point metal casting equipment, a variety of heat-resistant materials have been used to constitute the above mentioned portions for carrying out such things as molten metal transfer, supply and holding, for example pouring boxes, launders and troughs, holding furnace inner linings etc., floats, spouts, hot top rings etc., and among such material the calcium silicates have been widely used due to their excellent high heat insulating characteristics, small heat capacities and further their so-called non-wetting characteristics. However, the use of asbestos fiber which has been used since the past as a reinforcing fiber for such calcium silicate type materials has become difficult to use for well-known reasons. Hence the production of a product not containing such fibers, but still remaining characteristics equivalent to those of the product of the prior art reinforced with such fibers has been desired. Likewise, the replacement of the asbestos fiber with other fibers has also become necessary for calcium silicate molded products used as building materials or heat insulations, and the replacement with alkali resistant glass fiber has almost been successful and is being put into practice in those industries. However as far as a compositional material for casting equipment for low melting point metal is concerned, because of the special requirements for the material in this field, no satisfactory solution of this matter has been in sight. That is, since crystals composing a calcium silicate type molded body contains some crystalline water depending on various crystal forms, when such material is employed as a compositional material for casting equipment for low melting point metal as it is, the crystalline water is dehydrated being turned into steam and causes various troubles in the molten metal. Accordingly, it is necessary to eliminate the crystalline water and adsorbed water by heating at about 300-600.degree. C. for about 3-24 hours, but by such heat treatment the tensile strength of the alkali resistant glass fiber deteriorates to less than 1/2 of the original. Hence, physical properties such as elasticity, toughness, strength, etc. of the final product become unsatisfactory.
In order to resolve the problem of deterioration in physical strength of the reinforcing fiber due to heat treatment, the invention according to Japanese Patent Publication No. 57-49057, adopts a burning method wherein a calcium silicate molded body is produced from slurry comprising a mixture of lime and siliceous material having a CaO/SiO.sub.2 mole ratio of 0.6-1.2, a xonotlite previously prepare by hydrothermal synthesis, a fibrous wollastonite, a reinforcing fiber and water, and then burned. This method produces a calcium silicate molded body which has very little crystalline water from hydrothermal reaction by formulating large amount of unhydrous wollastonite and xonotlite which has little crystalline water hence the subsequent heat treatment can be accomplished in a short time, with only slight deterioration of the reinforcing fiber by the heat treatment. In this production method, if the calcium silicate crystal produced from lime and siliceous raw materials could be entirely low crystalline water containing xonotlite, the heat treatment could be extremely simplified or totally omitted. However, the attempt to make the products of hydrothermal reaction entire xonotlite require severe autoclave condition for the molded body causing a problem that the reinforcing fiber deteriorates and lowers its reinforcing effect. Consequently, even by this method a product having sufficient physical characteristics cannot be obtained.
For the reasons mentioned above, if a calcium silicate with insufficient physical strength and toughness is employed as a compositional material for low melting point metal casting equipment, the biggest problem is the large cracks occurred during casting operation. Needless to say, it is easy to crack widely in a material with inferior physical characteristics, however even in a material seemingly excellent in its physical characteristics also cracks generate due to the stress of the uneven temperature elevation caused by one side contact of the molten metal. This defect could bring about serious accidents such as leakage of molten metal or equipment destruction.
Simple insufficiency of physical strength can be build up operationally by the use of sufficiently thick material or the use of a reinforcing back up material. However, above mentioned cracks cannot be prevented by operational countermeasures.